1. Field of the Invention
The present invention relates to a titanium, and more particularly to a titanium casting. The titanium possesses improved castability and is suitable for use in biomedical implants and other applications.
2. Description of the Prior Art
Due to its lightweight, high strength-to-weight ratio, low elastic modulus, superior chemical corrosion resistance, and excellent mechanical properties at high temperature up to 550xc2x0 C., titanium and its alloys have been widely used on aerospace, chemical, sports, and marine industries. Their superior biocompatability also makes them ideal as the primary materials used in dental and osteological restorations or implants, such as artificial bone pins, bone plates, shoulders, elbows, hips, knees and other joints, and dental orthopraxy lines.
A number of methods for fabricating titanium and its alloys with a desired shape have been developed. Among these, precision casting is the most difficult. Precision casting has the advantage that the cast produced has a near net shape, which greatly decreases the titanium fabrication cost. Also, precision casting is particularly suitable for producing objects with a small volume, high size accuracy, and complicated shape, for example in dental and osteological fields. However, if the difficulty in precision casting could be solved and the cost reduced, titanium and its alloys could even be utilized in many other everyday products.
There are many factors that affect the process of the precision casting and the properties of castings. According to the research results issued by Luk et al., in Dent. Mater., 8, 89-99,1992, the factors includes alloy composition, alloy density, alloy surface tension, casting temperature, investment material type, mold temperature, casting machine type, casting surface area/volume ratio, and pouring angle. The castability test is the most frequently used method for assessing various titanium precision casting processes. Castability is the ability of a molten alloy to completely fill a mold space. Castability is a combination factor, and there is no international standard for assessing it today. Since castability is affected by many factors, researchers have designed various test methods in accordance with various cast patterns for assessing the castability. The cast patterns includes spiral wax molds, fibrous nylon lines produced by injection molding (Howard et al., JDR, 59, 824-830, 1980), saucer-like molds, cylindrical molds, rectangle sheets, nylon mesh, and taper molds (Mueller et al., J. of Prosth. Den., 69, 367, Abstr. 2072, 1993). A wax mold of a simulated crown has also been designed (Bessing et al., Acta Odontology Scandinavian, 44, 165-172, 1986).
Titanium is inherently difficult to cast due to its high melting point and high reactivity. Its low density is another problem in casting. Therefore, the improvement of casting process is the main issue of titanium precision casting. The casting machines used at present utilize argon as the protective atmosphere to prevent high temperature reactions. Induction or arc is used as the heat source in order to shorten melting time as well as lessen high temperature reactivity. At present, in order to increase the pouring force and to avoid casting defect caused by poor flowability of the molten metal, the titanium casting machines can be roughly divided into the centrifugal casting type, the vacuum-pressure type, and the centrifugal-vacuum pressure mixed type (Yoshiaki, Conference Paper, 1-7, Australia, 1995).
A number of patents improving the castability of metal and metal alloy by means of different casting methods and machines have been issued. U.S. Pat. No. 4,763,717 issued to Lajoye et al. discloses a method utilizing a compact assembly for melting and centrifugal casting of metals. The melting is accomplished entirely in a vacuum condition, thereby avoiding the formation of oxides or other compounds. U.S. Pat. No. 5,065,809 issued to Sato et al. discloses a method for casting titanium or a titanium-based alloy by fusing the metal in a vacuum furnace while introducing argon gas therein and casting the resultant melts in mold. U.S. Pat. No. 5,119,865 issued to Mae et al. discloses a method for centrifugal casting titanium or a titanium-based alloy by using a Cu-alloy mold. The mold body is made of a copper alloy satisfying the following relationship: Ts+0.3.rho..gtoreq.70 where Ts is the tensile strength (kg/mm2), and .rho. is the electrical conductance (% IACS). A cavity disposed in the mold body has a volume that is at most 30% of the volume of the mold body. U.S. Pat. Nos. 5,168,917 and 5,168,918 issued to Okuda et al. disclose a method for casting dental metals, which is characterized by positioning a dental metal ingot on the crucible, evacuating the casting chamber to vacuum, feeding a small amount of an inert gas at such a pressure as to induce arc discharge all over the upper surface of the ingot, thereby melting the ingot placed on the crucible by arc discharge from the arc electrode, pouring the thus obtained molten metal into a mold through its inlet, and immediately feeding an additional amount of the inert gas into the casting chamber to increase its internal pressure to a level suitable for casting. U.S. Pat. No. 5,193,607 issued to Demukai et al. discloses a method for precision casting titanium or titanium. The method includes establishing molten metal by induction heating in an assembly formed with water cooled copper segments disposed radially on the inside of an induction coil in a state insulated form each other and casting the molten metal into a permeable mold by vacuum casting. U.S. Pat. Nos. 5,267,602 and 5,392,842 issued to Volpe disclose a method for casting metal by melting it in a crucible over a hole too small to enable gravity to make the molten metal pass through the hole, after which increased pressure is applied to the metal to drive it through the hole into a mold. U.S. Pat. No. 5,626,179 issued to Choudhury et al. discloses a method for centrifugal casting titanium or a titanium-based alloy by using a reusable mold. The mold, at least in the area of the surface which comes in contact with the melt, consists of at least one metal selected from the group consisting of tantalum, niobium, zirconium, and/or their alloys.
Research has been conducted for improving the castability of metal and metal alloy by means of mixing various alloys. Asgar and Kaminski indicate that the alloy compositions of base metal alloys, high fusion noble metal alloys, or conventional type III gold alloys have a certain effect upon castability (Asgar et al., J. of Prosth.Dentistry, Vol. 54, No.1, 60-63, 1985; Kaminski et al., J. of Prosth Dentistry, Vol. 53, No.3, 329-332, 1985). Vincent et al. investigated the castability of five alloys, Thermocraft (atthey Garrett Pty. Ltd., Brisbane, Queensland, Australia), Degudent Universal (Degussa, Pforzheim, Germany), Victory (Unitek Corporation, Monrovia, Calif.), Ul-tratek (metals for Modern Dentistry, Inc., Danville, Calif.), and Wiron S (Bego, Bremen, Germany). The results indicated that castability is related to density and problems caused by low density may be solved by adjusting equipment, investment, and casting techniques to increase casting force (Vincent et al., J. of Prosth. Dent., 37, 527-536, 1977). Cohen et al. had studied the influence of beryllium on castability of Ni-Cr alloys and indicated that Be content exceeding 1% may be desirable in order to obtain optimal castability (Cohen et al., JDR, Abstr.609, 1991). Bessing et al. evaluated the castability of the simulated crowns of two low-gold alloys and two kinds of Ag-Pd alloys. It is found that the castability of the two low-gold alloys is comparable with that of the conventional type III gold alloys (Bessing et al., Swed. Dent. J., 16, 109-113, 1992). Most of the above-mentioned research was related to noble metals or its alloys and base metal alloys (such as nickel-chromium alloys). The casting of titanium and titanium is seldom mentioned.
The following are some related U.S. Patents about the alloying metals added to titanium or titanium. U.S. Pat. No. 4,830,823 issued to Nakamura discloses that it is preferable to cast a titanium containing aluminum at a ratio of 1.5 to 4.0% by weight and vanadium at a ratio of 1.0 to 3.0% by weight. In particular, the castings obtained by casting a xe2x80x9cTi-3Al-2.5V alloyxe2x80x9d containing aluminum at a ratio 3.0% by weight and vanadium at a ratio of 2.5% by weight exhibit physical characteristics equal to those incidental to the Ti-6Al-4V alloy and superior to that of pure titanium castings. In the case where the Ti-6Al-4V alloy itself is cast, superior characteristics incidental to said alloy are lost and as a result, it becomes hard and brittle, whereby it cannot be used as cast. U.S. Pat. No. 5,091,148 issued to Prasad discloses that the addition of 0.01 to 0.5 wt % of one or more alloying metals of ruthenium, rhodium, hafnium or strontium into a titanium-aluminum alloy (such as Ti5Al2Sn, Ti6Al7Nb, or Ti6Al4V) can increase the corrosion and tarnish resistance. However, aluminum and vanadium have adverse effect in human health and are controversial for use as biomedical implants.
The object of the present invention is to solve the above-mentioned problems and to provide a titanium having improved castabiliby and a decreased surface tension compared with pure titanium.
To achieve this object, the titanium of the present invention includes titanium and from about 0.01 to 3 percent by weight of an alloying metal selected from the group consisting of bismuth, silver, hafnium, tantalum, molybdenum, tin, niobium, chromium and copper based on the weight of the titanium.
The present invention also provides a casting cast from the titanium mentioned above.